You choose the target for Soul Seizer’s triggered ability when that ability triggers and goes on the stack. You choose whether or not to transform it when that ability resolves. If the creature is an illegal target by that time, the ability doesn’t resolve and none of its effects happen. You can’t have Soul Seizer transform.

1/22/2011

If Soul Seizer leaves the battlefield in response to its triggered ability, you won’t be able to transform it and it won’t return to the battlefield or become attached to the target.

1/22/2011

If Soul Seizer has any +1/+1 counters on it when it transforms, they remain on Ghastly Haunting. They won’t affect the power or toughness of the enchanted creature, however.

7/13/2016

For more information on double-faced cards, see the Shadows over Innistrad mechanics article (http://magic.wizards.com/en/articles/archive/feature/shadows-over-innistrad-mechanics).